The subject matter disclosed herein relates to sensors for measuring impedance and more particularly for sensors for measuring impedance caused by biological cells.
The electric cell substrate impedance spectroscopy (ECIS) method was pioneered by Giaever and Keese, and has been extensively studied for over two decades due to its simple structure, easy operation, and sensitive to many cell behaviors and properties. The name “impedance spectroscopy” is derived from the fact that the impedance is generally determined at different frequencies rather than just one. The measurements are generally performed using a small ac electric field over a wide frequency range (100 to 100 kHz). As cells attach on the electrodes, the insulating properties of the cells can be detected, since the cells contribute additional resistance to the circuit. A short time after the cells are seeded on a device, the cells form a monolayer and the impedance values are high. These impedance values will be stable over a period of time, as long as the cells are alive. When cells start to die, the cells lose the dielectric properties and there will be a decrease in the measured membrane impedance. Morphological information about the cells on the electrode surface could be extracted from the recorded impedance readings.
The impedance measurements can be correlated to cell size, attachment, growth, proliferation, stiffness and cell viability. The impedance values increase with increasing cell density and reaches equilibrium when the cells are confluent. Normal cell lines adhere more tightly to a surface in comparison to cancer cells. Apoptotic (dead) cells tend to detach from the sensing electrodes and are characterized by very low impedance values approaching zero. Thus, impedance spectroscopy of cell activity is a versatile and sensitive way to detect the response of the cells to a variety of biologically active agents.
At the moment, the most commonly used method to study the cell attachment and proliferations are via assays. This type of assays can only measure the end-point of cell behavior. End point measurements are tedious when a large number of points are required, as the entire treatment and preparation process has to be repeated for each measurement point. Alternative methods of obtaining impedance measurements of living cells is therefore desirable.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.